A new OEC cell has been constructed using improved temperature controls. Work has also progressed on the construction of a diffusion membrane system that would permit both concentrated Hb solutions and whole blood to be measured either from oxygenated to deoxygenated or the reverse. The effect of high hematocrit on the measured blood pH has been carefully examined and found to be due to the effect of the charge on the red cell on the glass electrode and not the reference junction potential. A complete physiochemical analysis of the glucose-hexokinase system has been carried out in order to explain the experimental results obtained with the differential pH apparatus.